The linear actuator is a common device having many different uses in a variety of fields. These actuators come in a variety of designs, depending on the application. The following discussion deals with the type having a motor with a rotatable output shaft on which is mounted a cam. The cam efficiently converts the rotary motion to linear motion.
One very common application for linear actuators of the cam-operated type is in opening and closing valves, and it is in this context that the invention will be explained. However, the reader should understand that I do not intend to limit the scope of the disclosure to valves only as the actuated devices. There are many different types of workpieces for which positioning is important, and where the invention to be described may have application.
In many of these positioning applications, it is important that the closure force be controllable or adjustable. (The term "closure force" will be used hereafter, but is meant to include the force required to open the valve as well.) By "controlling" closure force, is meant the ability to easily provide a particular value of force in an actuator. By "adjusting" closure force is meant the ability to easily change the closure force provided by a particular actuator either at the factory or in the field. When operating a valve for example, too great a force may damage the valve seat. Too little force may not close the valve securely. It is convenient to be able to generate differing closure forces from the same basic actuator design since this allows a wide range of applications with only a few different parts.
In certain prior art actuators, the practice is to use a spring to transmit the closure force, and by changing the stiffness of the spring, change the closure force. When a range of different values of closure force is required, this means that a different strength of spring is required for each closure force value. This leads to a relatively large number of different spring values to carry in the manufacturing inventory. Accordingly, it would be advantageous to use a design which eliminates the need for a number of different springs and at the same time allows for a number of different closure force values.